There are three manufacturers that I am aware of. Midway carries Pachmayr and Tipton; I have had best luck with the Tiptons. The Pachmayrs can be brittle; ocasionally the rim will shatter.
Regardless, you still have to rotate the case between strikes, as each strike flattens a section of the rim which then will not cushion another blow, at least not very well. You'll also have to replace each cap after every 20 strikes or so; I buy them buy the hundred.
If you do not have difficulty slipping empty brass into the chamber, and you do not mind the occasional carbon "clinker" getting tossed into the bore from the fired case, there is no real advantage to snap caps over empties.